The present invention relates generally to radio communications systems and, in particular, to antennas which can be incorporated into portable terminals and which allow the portable terminals to communicate within different frequency bands.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry's growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
Throughout the world, one important step in the advancement of radio communication systems is the change from analog to digital transmission. Equally significant is the choice of an effective digital transmission scheme for implementing the next generation technology, e.g., time division multiple access (TDMA) or code division multiple access (CDMA). Furthermore, it is widely believed that the first generation of Personal Communication Networks (PCNs), employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, etc., will be provided by, for example, cellular carriers using the next generation digital cellular system infrastructure.
To provide an acceptable level of equipment compatibility, standards have been created in various regions of the world. For example, analog standards such as AMPS (Advanced Mobile Phone System), NMT (Nordic Mobile Telephone) and ETACS and digital standards such as D-AMPS (e.g., as specified in EIA/TIA-IS-54-B and IS-136) and GSM (Global System for Mobile Communications adopted by ETSI) have been promulgated to standardize design criteria for radio communication systems. Once created, these standards tend to be reused in the same or similar form, to specify additional systems. For example, in addition to the original GSM system, there also exists the DCS1800 (specified by ETSI) and PCS1900 (specified by JTC in J-STD-007), both of which are based on GSM.
However, the most recent evolution in cellular communications services involves the adoption of additional frequency bands for use in handling mobile communications, e.g., for Personal Communication Services (PCS) services. Taking the U.S. as an example, the Cellular hyperband is assigned two frequency bands (commonly referred to as the A frequency band and the B frequency band) for carrying and controlling communications in the 800 MHz region. The PCS hyperband, on the other hand, is specified in the United States of America to include six different frequency bands (A, B, C, D, E and F) in the 1900 MHz region. Thus, eight frequency bands are now available in any given service area of the U.S. to facilitate communications services. Certain standards have been approved for the PCS hyperband (e.g., PCS1900 (J-STD-007), CDMA (IS-95) and D-AMPS (IS-136)), while others have been approved for the Cellular hyperband (e.g., AMPS (IS-54)).
Each one of the frequency bands specified for the Cellular and PCS hyperbands is allocated a plurality of traffic channels and at least one access or control channel. The control channel is used to control or supervise the operation of mobile stations by means of information transmitted to and received from the mobile stations. Such information may include incoming call signals, outgoing call signals, page signals, page response signals, location registration signals, voice channel assignments, maintenance instructions, hand-off, and cell selection or reselection instructions as a mobile station travels out of the radio coverage of one cell and into the radio coverage of another cell. The control or voice channels may operate in either an analog mode, a digital mode, or a combination mode.
The signals transmitted by a base station in the downlink over the traffic and control channels are received by mobile or portable terminals, each of which have at least one antenna. Historically, portable terminals have employed a number of different types of antennas to receive and transmit signals over the air interface. For example, monopole antennas mounted perpendicularly to a conducting surface have been found to provide good radiation characteristics, desirable drive point impedances and relatively simple construction. Monopole antennas can be created in various physical forms. For example, rod or whip antennas have frequently been used in conjunction with portable terminals. For high frequency applications where an antenna's length is to be minimized, another choice is the helical antenna. As seen in FIG. 1, a helical antenna allows the design to be shorter by coiling the antenna along its length.
In order to avoid losses attributable to reflections, antennas are typically tuned to their desired operating frequency. Tuning of an antenna refers to matching the impedance seen by an antenna at its input terminals such that the input impedance is seen to be purely resistive, i.e., it will have no appreciable reactive component. Tuning can, for example, be performed by measuring or estimating the input impedance associated with an antenna and providing an appropriate impedance matching circuit.
As described above, it will soon be commercially desirable to offer portable terminals which are capable of operating in widely different frequency bands, e.g., bands located in the 900 MHz region and bands located in the 1800 MHz region. Accordingly, antennas which provide adequate gain and bandwidth in both frequency bands will need to be employed in portable terminals in the near future. Several attempts have been made to create such dual band antennas.
For example, U.S. Pat. No. 4,571,595 to Phillips et al. describes a dual band antenna having a sawtooth shaped conductor element. The dual band antenna can be tuned to either of two closely spaced apart frequency bands (e.g, centered at 915 MHz and 960 MHz). This antenna design is, however, relatively inefficient since it is so physically close to the chassis of the mobile phone.
U.S. Pat. No. 4,356,492 to Kaloi describes a multi-band microstrip antenna including a plurality of separate radiating elements which operate at widely separated frequencies from a single common input point. However, these radiating elements are directly connected with each other and require a ground plane which fully covers the opposite side of a dielectric substrate from such radiating elements. Thus, the design of Kaloi is impractical for monopole antenna applications and, in fact, functions in a completely different manner.
U.S. Pat. No. 5,363,114 to Shoemaker discloses a planar serpentine antenna which includes a generally flat, non-conductive carrier layer and a generally flat radiator of a preselected length arranged in a generally serpentine pattern secured to the surface of the carrier layer. One form of this antenna has a sinuous pattern with radiator sections in parallel spaced relation to provide dual frequency band operation. However, it is seen that the two frequencies at which resonance takes place involves the length of each radiator section and the total length between first and second ends. While this arrangement may be suitable for its intended purpose, it is incapable of operating in the manner of a monopole antenna.
Accordingly, it would be desirable for an antenna design to have the desirable characteristics of a monopole antenna and be relatively compact in size for usage in portable terminals. Moreover, it would further be desirable that such an antenna be tuned to two (or more) frequency bands for compatibility with various, overlapping radio communication systems.